Controlling yarn tension is very important in order to achieve maximum effectiveness in many weaving operations, such as insertion of the weft yarn in a high-performance loom, like the modern shuttleless looms, or feeding of a group of yarns to a warping machine, or still other operations. In all these activities, the way in which the yarn is drawn off is such that yarn tension may vary considerably. A particular case is insertion of weft yarn in a shuttleless loom, where the yarn is fed in at extremely high and extremely variable speed, causing tension peaks in the yarn that may even result in the yarn breaking.
Mechanical devices are known which control yarn braking in relation to yarn tension, so as to maintain a substantially constant tension in the yarn, far from very high values and exempt from sharp variations. These are equipped with brakes that react mechanically to the tension existing at any moment in the yarn, so that the braking force exerted by the brake on the yarn decreases as the tension increases and, conversely. Braking is mechanically modulated in relation to the yarn tension value. Mechanical devices generally have a slow response time in detecting variations of tension occurring at high frequency. These drawbacks derive from the fact that the yarn tension is detected to modulate braking by using mechanical parts engaging with the yarn and that are prone to move during the yarn tension variations. Though of very small mass, these mechanical parts nevertheless have an inertia and an elasticity that are such that, they interact dynamically with the yarn exchanging kinetic and elastic energy with it, especially in cases of a sudden variation in yarn tension. These dynamic variations slow down the response and diminish the efficaciousness of these devices to control the brake and, as a result, the yarn tension. Moreover, the dynamic interaction of mechanical parts and yarn has an unsettling effect on existing yarn status, and may thus considerably modify yarn tension, especially when the tension varies extremely rapidly and the dynamic interactions between these mechanical parts and the yarn occur continuously. These known devices are, in addition to their slow speed of reaction to tension variations, never completely passive with respect to the yarn in detecting its tension and always alter its value to a certain extent, thereby reducing efficaciousness of braking modulation. Furthermore, these known devices do not measure the absolute or numeric value of yarn tension but are limited to detecting relative variations of tension in order to control yarn braking.
As known devices do not possess a feature whereby the effective tension value is visualised this prevents these devices from being set accurately and permits only an empirical regulation (generally performed `by feel` by an operator).
U.S. Pat. No. 5,316,051 shows a truncated cone-shaped cap fitted on the front end of the drum of a yarn feeder to press directly on the yarn coming from the reserve and passing between cap and drum. This brake exerts a braking force on the yarn that is responsive to the tension in the yarn when exiting from the brake itself. In fact, this exiting yarn tension gives rise to a tension component oriented axially and apt to act retroactively on the cap for being subtracted from the braking force exerted by the cap on the yarn. The axial tension component causes a reduction of the braking force. Therefore, if yarn tension tends to rise, the brake reduces its braking force proportionally. This brake reduces yarn tension peaks and smoothes the tension pattern, but cannot completely cease from braking on the yarn, i.e. the yarn can never be disengaged from the brake completely to reduce yarn tension as much as possible. This brake cannot entirely nullify the tension at the brake outlet. The yarn always remains engaged with and is thus braked by the cap.
Devices are known to control yarn tension that are used on a yarn feeder, and include yarn tension gauge attached to the eyelet coaxial with the feeder drum. Tension measurement is obtained by measuring the variation of parameters due to axial displacement of the eyelet, because of the force dependent on yarn tension. The eyelet is thus left free to move in the axial direction under the action of this force. These devices have the tendency to exchange kinetic and/or elastic energy with the yarn, thus leading to the risk of the eyelet starting to vibrate. These inconveniences mean that tension measurement is generally affected by error and is thoroughly unreliable when yarn is drawn off from the drum at extremely high and rapidly varying speed.
One object of this invention is to provide a yarn tension control device having a very high response speed and high precision in measuring yarn tension.
A further task of the present invention is to provide a yarn tension control device that visualises the effective tension values, so that the latter may be of use to an operator for convenient setting of the desired tension threshold values and also make it possible to subsequently control deviation in time of the tension from the desired values, so that action may be taken if the deviations are found to be anomalous.
Another object of this invention is to provide a yarn tension control device which, as well as measuring yarn tension, also has a very high response speed and high precision in varying braking force on the yarn, so as to intervene in an effective and timely manner to correct yarn tension and maintain it substantially within a programmed pattern, especially when the yarn is subjected to stresses varying extremely rapidly while it proceeds along its path and which is also able, as required, to disengage from the yarn and thus completely annul braking on the latter.
According to another aspect of the invention, the movable guide element engaging the yarn is associated with elastic yarn retracting means, which can be regulated manually and which are arranged between the movable guide element and the yarn tension sensor means in order to retract the yarn and keep it constantly under tension, preventing it from becoming slack.
According to another aspect of the invention, the motor of the yarn tension sensor means may be used for positive driving of the guide element so as to retract and tension the yarn in cases where it tends to become slack as, for example, at the end of a cycle of weft yarn insertion in a shuttleless loom.
The device of the invention is particularly advantageous on shuttleless looms due to its capacity to control weft yarn braking during the insertion stage as a function of the tension effectively acting in the yarn and not, as is the case in known systems, in a way dependent on or synchronised with the loom cycle. The inventive concept permits the control of yarn tension at the outlet of a yarn feeder, during the feeding of yarn at high speed into a weaving machine. The yarn passes through an eyelet at high speed and exerts a force on the eyelet that is dependent on the tension of the yarn.
The device and method offer considerable advantages, such as that of not altering the path normally followed by the yarn in the yarn feeder as it unwinds and that of being able to obtain a yarn tension measurement from a tension component apt to assume a significant value, i.e. similar to that of actual tension of the yarn. In fact, the device of the invention utilises to advantage the deviation that the yarn undergoes in passing through the eyelet coaxial with the drum on its way to the weaving machine. This arrangement of the eyelet with respect to the drum is such as to produce a very high angle of deviation of the yarn, in correspondence with the eyelet, as the yarn reaches the eyelet along a path that is radial with respect to the drum, coming from its outer cylindrical surface, whereon the yarn reserve is located. The effect of said high deviation is that the force the yarn exerts on the eyelet and which is proportional to both the tension to be measured and to the deviation undergone by the yarn, assumes a very significant value, similar to that of the tension and apt to permit a precise and reliable measure of the said tension.
A preferred embodiment of the invention relates to a yarn tension control device including a guide element which engages the yarn and is subject to a force proportional to the tension of the yarn, and a yarn tension sensor means which detects the tension in the yarn through the guide element. The yarn tension sensor means includes a control circuit, electromagnetic control means to which the control circuit supplies current and which is provided with a moveable control element operable electromagnetically and a position sensor. The moveable control element moves along with the moveable guide element, and the position sensor emits a signal indicative of displacements of the moveable control element from a determined measuring position and corresponding to displacements of the guide element caused by said force proportional to the tension of yarn. The control circuit supplies the electromagnetic control means with current to generate a magnetic control force for the control element for maintaining same substantially motionless in the determined measuring position during variations of tension of yarn. This current supplied to the electromagnetic control means is indicative of the yarn tension, and the current is measured by the tension sensor means to generate a signal indicative of the tension.